Superconductivity-enhanced phonon angular momentum

We theoretically investigate the properties of phonon angular momentum in the superconducting state, using fulleride compounds in an external magnetic field as a model system. The electron orbital angular momentum injected by an external field is transferred to the phonon subsystem via electron--phonon coupling. We show that this field-induced phonon angular momentum is significantly enhanced and undergoes a sign reversal upon entering the superconducting state. In the normal state, the dominant energy scale governing the response function is the electronic bandwidth $D$. In the superconducting state, the phonon energy scale $\omega_1$ enters the denominator, leading to an enhancement of order $D/\omega_1$. The observed sign change in the response can be explained by the competition between Fermi surface and Fermi volume contributions.